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Do Android phones have a BIOS?

D

Dammitbobby3

Lurker
This is a question I've always had.

How does the boot system work in Android smartphones, and do they have a BIOS like a pc?

If there is no separate BIOS or boot ROM then what part of the system is working when A: the phone is powered off and the battery is charging or dead and battery animation appears on screen?

And B: when the device first powers on and displays the OEM splash screen, and prepares to boot Android?
 
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The bootloader is the thing that manages the loading of the operating system, or (when instructed, or if the OS is unavailable) the loading of the recovery module.

As for A, I'm not certain what module provides that. Not all phones even have that, though I do know that where they do different manufacturers keep the animation in different places.
 
No that would refer as the device OS kernel.
Here's something you can wrap your head around awhiles.
  • first-stage bootloader runs
    • it detects if a special key is held, and can launch the recovery image, or the 'fastboot' bootloader
  • eventually, a kernel is loaded into RAM (usually with an initrd)
    • normally, this will be the kernel from the 'boot' flash partition.
kernel
  • the kernel boots
    • core kernel initialization
      • memory and I/O areas are initialized
      • interrupts are started, and the process table is initialized
    • driver initialization
    • kernel daemons (threads) are started
    • root file system is mounted
    • the first user-space process is started
      • usually /init (note that other Linux systems usually start /sbin/init)
user space
  • the kernel runs /init
    • /init processes /init.rc and /init.<machine_name>.rc
    • dalvik VM is started (zygote). See Android Zygote Startup
    • several daemons are started:
      • rild - radio interface link daemon
      • vold - volume daemon (media volumes, as in file systems - nothing to do with audio volume)
  • the system_server starts, and initializes several core services
  • the activity manager starts core applications (which are themselves dalvik applications)
    • com.android.phone - phone application
    • android.process.acore - home (desktop) and a few core apps.
  • other processes are also started by /init, somewhere in there:
    • adb
    • mediaserver
    • dbus-daemon
    • akmd
Tools for analyzing Android Bootup
01-01 00:00:08.396 I/boot_progress_start( 754): 12559
01-01 00:00:13.716 I/boot_progress_preload_start( 754): 17879
01-01 00:00:24.380 I/boot_progress_preload_end( 754): 28546
01-01 00:00:25.068 I/boot_progress_system_run( 768): 29230
01-01 00:00:25.536 I/boot_progress_pms_start( 768): 29697
01-01 00:00:25.958 I/boot_progress_pms_system_scan_start( 768): 30117
01-01 00:00:40.005 I/boot_progress_pms_data_scan_start( 768): 44171
01-01 00:00:45.841 I/boot_progress_pms_scan_end( 768): 50006
01-01 00:00:46.341 I/boot_progress_pms_ready( 768): 50505
01-01 00:00:49.005 I/boot_progress_ams_ready( 768): 53166
01-01 00:00:52.630 I/boot_progress_enable_screen( 768): 56793

    • or this: 'adb logcat -d | grep preload'
10-15 00:00:17.748 I/Zygote ( 535): ...preloaded 1873 classes in 2438ms.
10-15 00:00:17.764 I/Zygote ( 535): ...preloaded 0 resources in 0ms.
10-15 00:00:17.772 I/Zygote ( 535): ...preloaded 15 resources in 7ms.

  • Bootchart - see Using Bootchart on Android
  • strace is pretty handy also, to see the timings for system calls from a process as it runs
    • You can use strace as a wrapper for a program in init.rc, and save the results to a file
    • Use -f to follow sub-processes
    • Use -tt to get detailed timestamps for syscalls
    • Use -o to output the data to a file
Here is an example of using strace to follow the startup of zygote, and the apps that are forked from it.

Replace:

service zygote /system/bin/app_process -Xzygote /system/bin --zygote --start-system-server

with

service zygote /system/xbin/strace -f -tt -o /cache/debug/boot.strace /system/bin/app_process -Xzygote /system/bin --zygote --start-system-server

Here is some sample data:

$ head boot.strace
571 00:00:11.389939 execve("/system/bin/app_process", ["/system/bin/app_process", "-Xzygote", "/system/bin", "--zygote", "--start-system-server"], [/* 15 vars */]) = 0
571 00:00:11.658878 brk(0) = 0x804b000
571 00:00:11.659048 mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x77f9a000
571 00:00:11.659169 readlink("/proc/self/exe", "/system/bin/app_process", 4096) = 23
571 00:00:11.659339 access("/etc/ld.so.preload", R_OK) = 0
571 00:00:11.659440 open("/etc/ld.so.preload", O_RDONLY) = 3
571 00:00:11.659548 fstat64(0x3, 0x7fa76650) = 0
571 00:00:11.659887 mmap2(NULL, 36, PROT_READ|PROT_WRITE, MAP_PRIVATE, 3, 0) = 0x77f99000
571 00:00:11.659970 close(3) = 0
571 00:00:11.660071 open("/lib/libc_sse.so", O_RDONLY) = 3

Please note that writing the strace data takes extra time. For long sequences of very fast syscalls (such as when the timezone file is being read) the overhead of strace itself exaggerates the timings in the trace. Use the timing information with caution.

Notes on the Android startup procedure
Overview
See "Android Initialization Process" at: http://blog.chinaunix.net/u2/85805/showart_1421736.html(this address is not work), using http://blog.chinaunix.net/space.php?uid=7788581&do=blog&id=2558375instead.

strace
http://benno.id.au/blog/2007/11/18/android-runtime-strace

Interaction of different processes on application initialization
Talking about Android Process - http://blog.csdn.net/mawl2002/archive/2009/06/24/4295905.aspx

Improving Bootup Time presentation
See Improving Android Boot Time - notes and material for a talk at LinuxCon North America.
 
Many thanks for this, it's something I've wondered about for a long time. IF (and it's a big if), I've understood this, then Android devices do not have a BIOS/UEFI in the same way as Intel/AMD PCs.
 
So what portion of the boot process is it when your phone does a bootloop? Like you see the carrier or Samsung logo then it just turns off and back on and process repeats?
 
Usually it's trying to load the Android operating system and failing. There can be many reasons for that, from corrupted system data (which you could fix with a factory reset, possibly) through to actual corruption of the ROM (e.g. the first time I experienced a bootloop it was when I did some trivial theming of a ROM - which is why anyone modding a phone should always take a nandroid backup before flashing anything, no matter how trivial).
 
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